cpython/Doc/library/asynchat.rst

:mod:`asynchat` --- Asynchronous socket command/response handler
================================================================

.. module:: asynchat
   :synopsis: Support for asynchronous command/response protocols.
.. moduleauthor:: Sam Rushing <rushing@nightmare.com>
.. sectionauthor:: Steve Holden <sholden@holdenweb.com>


This module builds on the :mod:`asyncore` infrastructure, simplifying
asynchronous clients and servers and making it easier to handle protocols
whose elements are terminated by arbitrary strings, or are of variable length.
:mod:`asynchat` defines the abstract class :class:`async_chat` that you
subclass, providing implementations of the :meth:`collect_incoming_data` and
:meth:`found_terminator` methods. It uses the same asynchronous loop as
:mod:`asyncore`, and the two types of channel, :class:`asyncore.dispatcher`
and :class:`asynchat.async_chat`, can freely be mixed in the channel map.
Typically an :class:`asyncore.dispatcher` server channel generates new
:class:`asynchat.async_chat` channel objects as it receives incoming
connection requests.


.. class:: async_chat()

   This class is an abstract subclass of :class:`asyncore.dispatcher`. To make
   practical use of the code you must subclass :class:`async_chat`, providing
   meaningful :meth:`collect_incoming_data` and :meth:`found_terminator`
   methods.
   The :class:`asyncore.dispatcher` methods can be used, although not all make
   sense in a message/response context.

   Like :class:`asyncore.dispatcher`, :class:`async_chat` defines a set of
   events that are generated by an analysis of socket conditions after a
   :cfunc:`select` call. Once the polling loop has been started the
   :class:`async_chat` object's methods are called by the event-processing
   framework with no action on the part of the programmer.

   Two class attributes can be modified, to improve performance, or possibly
   even to conserve memory.


   .. data:: ac_in_buffer_size

      The asynchronous input buffer size (default ``4096``).


   .. data:: ac_out_buffer_size

      The asynchronous output buffer size (default ``4096``).

   Unlike :class:`asyncore.dispatcher`, :class:`async_chat` allows you to
   define a first-in-first-out queue (fifo) of *producers*. A producer need
   have only one method, :meth:`more`, which should return data to be
   transmitted on the channel.
   The producer indicates exhaustion (*i.e.* that it contains no more data) by
   having its :meth:`more` method return the empty string. At this point the
   :class:`async_chat` object removes the producer from the fifo and starts
   using the next producer, if any. When the producer fifo is empty the
   :meth:`handle_write` method does nothing. You use the channel object's
   :meth:`set_terminator` method to describe how to recognize the end of, or
   an important breakpoint in, an incoming transmission from the remote
   endpoint.

   To build a functioning :class:`async_chat` subclass your  input methods
   :meth:`collect_incoming_data` and :meth:`found_terminator` must handle the
   data that the channel receives asynchronously. The methods are described
   below.


.. method:: async_chat.close_when_done()

   Pushes a ``None`` on to the producer fifo. When this producer is popped off
   the fifo it causes the channel to be closed.


.. method:: async_chat.collect_incoming_data(data)

   Called with *data* holding an arbitrary amount of received data.  The
   default method, which must be overridden, raises a
   :exc:`NotImplementedError` exception.


.. method:: async_chat.discard_buffers()

   In emergencies this method will discard any data held in the input and/or
   output buffers and the producer fifo.


.. method:: async_chat.found_terminator()

   Called when the incoming data stream  matches the termination condition set
   by :meth:`set_terminator`. The default method, which must be overridden,
   raises a :exc:`NotImplementedError` exception. The buffered input data
   should be available via an instance attribute.


.. method:: async_chat.get_terminator()

   Returns the current terminator for the channel.


.. method:: async_chat.push(data)

   Pushes data on to the channel's fifo to ensure its transmission.
   This is all you need to do to have the channel write the data out to the
   network, although it is possible to use your own producers in more complex
   schemes to implement encryption and chunking, for example.


.. method:: async_chat.push_with_producer(producer)

   Takes a producer object and adds it to the producer fifo associated with
   the channel.  When all currently-pushed producers have been exhausted the
   channel will consume this producer's data by calling its :meth:`more`
   method and send the data to the remote endpoint.


.. method:: async_chat.set_terminator(term)

   Sets the terminating condition to be recognized on the channel.  ``term``
   may be any of three types of value, corresponding to three different ways
   to handle incoming protocol data.

   +-----------+---------------------------------------------+
   | term      | Description                                 |
   +===========+=============================================+
   | *string*  | Will call :meth:`found_terminator` when the |
   |           | string is found in the input stream         |
   +-----------+---------------------------------------------+
   | *integer* | Will call :meth:`found_terminator` when the |
   |           | indicated number of characters have been    |
   |           | received                                    |
   +-----------+---------------------------------------------+
   | ``None``  | The channel continues to collect data       |
   |           | forever                                     |
   +-----------+---------------------------------------------+

   Note that any data following the terminator will be available for reading
   by the channel after :meth:`found_terminator` is called.


asynchat - Auxiliary Classes
------------------------------------------

.. class:: fifo(list=None)

   A :class:`fifo` holding data which has been pushed by the application but
   not yet popped for writing to the channel.  A :class:`fifo` is a list used
   to hold data and/or producers until they are required.  If the *list*
   argument is provided then it should contain producers or data items to be
   written to the channel.


   .. method:: is_empty()

      Returns ``True`` if and only if the fifo is empty.


   .. method:: first()

      Returns the least-recently :meth:`push`\ ed item from the fifo.


   .. method:: push(data)

      Adds the given data (which may be a string or a producer object) to the
      producer fifo.


   .. method:: pop()

      If the fifo is not empty, returns ``True, first()``, deleting the popped
      item.  Returns ``False, None`` for an empty fifo.


.. _asynchat-example:

asynchat Example
----------------

The following partial example shows how HTTP requests can be read with
:class:`async_chat`.  A web server might create an
:class:`http_request_handler` object for each incoming client connection.
Notice that initially the channel terminator is set to match the blank line at
the end of the HTTP headers, and a flag indicates that the headers are being
read.

Once the headers have been read, if the request is of type POST (indicating
that further data are present in the input stream) then the
``Content-Length:`` header is used to set a numeric terminator to read the
right amount of data from the channel.

The :meth:`handle_request` method is called once all relevant input has been
marshalled, after setting the channel terminator to ``None`` to ensure that
any extraneous data sent by the web client are ignored. ::

   class http_request_handler(asynchat.async_chat):

       def __init__(self, sock, addr, sessions, log):
           asynchat.async_chat.__init__(self, sock=sock)
           self.addr = addr
           self.sessions = sessions
           self.ibuffer = []
           self.obuffer = b""
           self.set_terminator(b"\r\n\r\n")
           self.reading_headers = True
           self.handling = False
           self.cgi_data = None
           self.log = log

       def collect_incoming_data(self, data):
           """Buffer the data"""
           self.ibuffer.append(data)

       def found_terminator(self):
           if self.reading_headers:
               self.reading_headers = False
               self.parse_headers("".join(self.ibuffer))
               self.ibuffer = []
               if self.op.upper() == b"POST":
                   clen = self.headers.getheader("content-length")
                   self.set_terminator(int(clen))
               else:
                   self.handling = True
                   self.set_terminator(None)
                   self.handle_request()
           elif not self.handling:
               self.set_terminator(None) # browsers sometimes over-send
               self.cgi_data = parse(self.headers, b"".join(self.ibuffer))
               self.handling = True
               self.ibuffer = []
               self.handle_request()